pedro2012:
ok,thanks
but in my programme i already read from the file Srings,so now i have Srings and i want to convert them to Float.
i don't know if you mean that i need to convert first from String to char and after to float ??
If you are using the "String" from the library, you can convert that to char with toCharArray() - Arduino Reference otherwise you are just calling string a char array 